Projection exposure systems for microlithographic manufacturing processes for micro-structured or nano-structured components in electrical engineering and micro-mechanics commonly include an illumination system by which a reticle can be illuminated, and a projection lens with which a reduced image of the structure of the illuminated reticle is projected onto a corresponding substrate. For the purpose of achieving high resolution, it is desirable that the reticle be illuminated with oblique incident light. Therefore it is common for illumination settings to be chosen in which, in one pupil plane of the illumination system, illumination poles are provided outside the optical axis (such as annular illumination settings, dipole illumination settings or quadrupole illumination settings). To be able to produce such illumination settings, components are used in the illumination system that can be described as pupil-defining elements and can effect a corresponding light intensity distribution in the pupil plane, such as, for example, a dipole distribution, quadrupole distribution or annular intensity distribution. In addition to diffractive optical elements (DOEs), microlens arrays, or computer-generated holograms (CGHs), micro-mirror arrays (MMAs) can serve as pupil-defining elements.
Although such illumination settings yield advantages in terms of resolution, one drawback is that very high surface energy densities can be generated in the pupil planes or planes conjugated thereto and planes of the illumination system near the pupil on account of the intensity concentration of the light. For optical elements near the pupil, this means high-energy surface stress (high surface power density). This can lead to damage in the materials used for the optical elements. If an ArF laser operating at a wavelength of 193 nanometres is used, for example, for projection exposure systems, the chemical bonds of the quartz material commonly employed for the optical elements may rupture. This can lead to the formation of so-called colour centres which absorb light.
In addition to this damage, further damage can occur, such as changes in the density of the material. Both an increase in density (compaction) and a reduction in density (rarefaction) can take place. When optical materials are exposed to high energy stress, micro-channels can also form, in which mechanical destruction of the material is present. Accordingly, compaction can lead to stress-induced birefringence or polarization-induced birefringence.
The damage done to the optical materials is generally related to the energy density or the surface stress, but the behaviour is not necessarily linear. Thus, types of damage in certain optical materials can occur only once a certain threshold is exceeded, while other types of damage can occur at lower energy densities, with extensive damage leading to appreciable impairment being observed only at higher stresses. In addition, the damage can also increase with an increase in duration of the stresses. As a result, with each additional energy stress (e.g., in the form of a laser pulse), additional damage occurs, so that the damage can add up with the number of laser pulses until a permissible limit is exceeded.
In certain known systems, the power of the light source, i.e. the laser, is adjusted such that, for the proposed illumination setting, the surface stresses on the optical elements or the energy densities in the projection exposure system are below the critical values, so that damage to individual optical elements or the projection exposure system can be avoided. The associated restriction on the radiant power of the light source can lead to a lower throughput, as the substrates to be exposed involve longer exposure to the exposure beam, and also can render the projection exposure systems inflexible in use, as the radiant power needs to be adjusted for every illumination setting.